The production of copper foil by electrochemical processes involves the use of a cell containing an anode, a cathode, an electrolyte solution containing copper ions and sulfate ions, and a source of current. Through the application of voltage between the anode and the cathode the deposition of copper is effected on the cathode surface.
The production of electrodeposited copper foil begins with the feed stock which can be copper shot, wire, recycled copper, etc. The copper feed stock is dissolved in sulfuric acid to form the electrolyte solution. The process can be accelerated by raising the temperature of the solution and by bubbling air through it. The resulting copper sulfate solution is then purified in order to ensure that high purity copper sulfate required for the production of foil is generated. Various types of agents for controlling the properties of the foil such as animal glue and thiourea can be added to the electrolyte solution.
The electrolyte solution is pumped into an electroforming cell, and with the application of voltage between the anode and cathode, the electrodeposition of copper takes place. Typically the process involves using cylindrical cathodes that may be of varying diameters and widths. The anodes that are used conform to the curvature of the cathodes so as to maintain constant separation between the two.
The qualities and characteristics of the copper foil that is produced are a function of many parameters such as current density, temperature, substrate material, solution agitation and electrolyte solution composition. A problem that has persisted in the art is that to date it has not been possible to produce low-profile copper foils with high ultimate tensile strengths and high elevated temperature elongations. The common practice of adding chloride ions to yield ductile electrodeposits induces a decrease in ultimate tensile strength. When chloride ions are present in electrolyte solutions at common concentrations of 50 ppm or higher it is not possible to increase the ultimate tensile strength and high-temperature elongation to desired levels by changes in, for example, the concentration of animal glue or thiourea, or by changes in the current density.
Lakshmanan et al, "The Effect of Chloride Ion in the Electrowinning of Copper", Journal of Applied Electrochemistry 7 (1977) 81-90, discloses that the effect of chloride ion concentration on copper electrodeposition is dependent on the operating current density. At lower current density values the ridge type growth structure orientation is favored for additive-free electrolytes. At high current density values pyramidal growth orientation is favored for additive-free electrolytes. The addition of chloride ion to the 10 ppm level lowers the overvoltage and thus promotes ridge type oriented deposits. As the current density is increased to 40 amps per square foot, the pyramidal growth structure is again favored. The article indicates that the current densities that were tested ranged from 15 to 40 amps per square foot.
Anderson et al, "Tensile Properties of Acid Copper Electrodeposits", Journal of Applied Electrochemistry, 15 (1985) 631-637, discloses that the chloride ion concentration in an acid copper plating bath influences the ultimate tensile strength and elongation of the foil produced therefrom. The article indicates that at the current densities tested, acid copper plating baths require the presence of chloride ions to provide a ductile copper deposit. The current densities reported in the article ranged from 20 to 50 mA/cm.sup.2 [18.58 to 46.45 amps per square foot]. Chloride ion concentrations in the range of 0 to 100 ppm are reported.
U.S. Pat. No. 2,475,974 discloses a process for making copper deposits having tensile strengths of about 60,000 to about 73,000 psi and elongations of 6% to 9% using a copper plating solution containing triethanolamine.
U.S. Pat. No. 2,482,354 discloses a process for making copper deposits having tensile strengths of about 65,000 to about 90,000 psi and elongations of 8% to 12% using a copper plating solution containing triisopropanolamine.